Natural gas is a hydrocarbon gas mixture that is generally used as a source of energy. Natural gas includes mostly methane (CH4) in high concentrations, such as about 85% vol. for instance, with the balance of the gas stream including gases such as ethane, propane, higher hydrocarbon components, a small proportion of water vapor, nitrogen and/or carbon dioxide. Other components such as mercury, hydrogen sulfide and mercaptan can also be present in lower concentrations. Variants are possible.
Natural gas can be compressed and transported in gas pipelines but it can also be converted from its primary gas form to a liquid form at cryogenic temperatures for ease of storage and transportation. Liquefied natural gas (LNG) takes considerably less volume than natural gas in a gaseous state. This makes LNG more cost efficient to transport over long distances where natural gas pipelines do not exist.
LNG is increasingly used as an alternative fuel for transportation since it offers many advantages over other available kinds of fuel. For instance, it is an alternative fuel cleaner than other fossil fuels, with lower emissions of carbon and lower particulate emissions per equivalent distance traveled. LNG is also generally more energy efficient and provides a significant increase in the useful life of the engines. However, despite all its advantages, the widespread use of LNG in transportation faces several limitations due in most part to a lack of availability. There are a limited number of LNG production plants and a corresponding limited number of distribution points, i.e. fueling stations, particularly outside densely populated areas. Still, transporting LNG over long distances in relatively small quantities to supply remote fueling stations lowers environmental and economic benefits of LNG.
Natural gas is only one among a number of different possible sources of methane gas required for the production of LNG. For instance, landfill sites and anaerobic digesters can each generate significant amounts of biogas which contains methane gas, generally in concentration ranging from about 40 to 65% vol. under favorable operating conditions. Other gases that are mainly present in biogas include carbon dioxide, generally in concentration up to about 50% vol. of the gas stream, and nitrogen in concentration generally varying from a few percent to about 30% vol. of the gas stream. Other gases possibly present in smaller concentrations include oxygen, generally in concentration up to about 3% vol. of the gas stream, and hydrogen sulfide in concentration that are generally up to about 0.5% vol. of the gas stream. Other components can be present in even smaller concentrations, such as siloxanes, mercury, volatile organic carbons (VOC) and mercaptan. These compositions and concentrations are only examples. Variants are possible. Biogas originating from a landfill site or an anaerobic digester is generally saturated in water at the pressure and temperature conditions occurring at the capture points.
The methane gas fraction contained in biogas can be transformed into Liquefied Methane Gas (LMG). LMG can provide an equivalent to LNG in terms of quality and energy content. Thus, one could use LMG instead of LNG at fueling stations. This is particularly useful since biogas can be obtained locally almost anywhere, particularly from municipal landfill sites. Transforming biogas into LMG using small distributed production plants would then be highly desirable since this will promote an increase in the total number of fueling stations and solve supply issues in remote areas. It can also offer significant environmental and economic benefits over burning biogas in gas flares or releasing unburned biogas directly into the atmosphere.
Despite the fact that biogas is available almost everywhere and can be a very suitable low-cost alternative to natural gas as a source of methane gas, biogas is still rarely used for the production of LMG. This is due in most part to numerous challenges associated with the transformation of the methane gas fraction contained in biogas into LMG and that are unique to biogas. One of these challenges is the unpredictability of the biogas in terms of its total mass flow rate and the proportion of the methane gas fraction therein, particularly when biogas is captured in a landfill site. The concentration of the methane gas in the biogas collected from a landfill site may sometimes be insufficient to transform it into LMG. Air infiltrations can also lower the concentration and make the methane gas feed stream difficult to treat before entering a LMG production plant. Both situations may even occur simultaneously.
In landfill sites, the biogas composition and the methane gas concentration constantly fluctuate over time due to environmental factors, such as atmospheric pressure and temperature to name just a few. Cold weather conditions can also cause some collector pipes to freeze, thereby limiting biogas capture rate. Fluctuations also occur throughout the years since the decay of the organic matter within the landfill site will naturally diminish if no new waste materials is added. The water content within the organic matter may also diminish over time and cause the methane gas yield to drop.
In anaerobic digesters, the biogas composition and the methane gas concentration will often depend on the quality of the waste material supplied therein and their temperature. For instance, the methane gas concentration in the biogas tend to be lower under cold weather conditions. Other factors may exist. Hence, the biogas coming out of anaerobic digesters can fluctuate as well.
Existing LMG production plants are almost always custom designed and they rely on a methane gas source that is substantially stable. They are typically designed to provide a constant output capacity or to provide a capacity within a restricted range so that even a small LMG production plant requires a minimum mass flow rate of methane gas at any given time to be economically feasible and this can often be difficult to obtain. LMG production plants also often require many hours to restart after an interruption in order to reach their optimum production conditions. Thus, having an uninterrupted operation is thus highly desirable.
Notwithstanding the environmental factors, a LMG production plant can experience a methane gas shortage if they share the methane gas yield from a same biogas-generating site with another existing waste-to-energy project. For instance, if a greenhouse uses biogas for heating, the quantity of remaining methane gas available for the LMG production plant can be insufficient during certain parts of the year, given the fact that the heating requirements are the highest during cold weather conditions and this often coincides with a decrease in the methane gas yield. This problem may prevent the installation of the LMG production plant and the surpluses of methane gas coming out of the biogas-generating site will not become useful energy. Waste-to-energy facilities are generally not scalable and even an increase in the methane gas yield may not justify using the surplus in a LMG production plant. Significant increases in the methane gas yield can take years to happen. The same situation can happen when a new landfill site is opened. The biogas generation may take years to reach a certain level and the methane gas yield is not proven in advance.
Since the volume of biogas and the mass flow rate of its methane gas fraction continuously fluctuate, it can be desirable to rely on an alternative source of methane gas to compensate for the shortages. The methane gas yield can even stop without warning and it may be necessary to rely solely on the alternative source of methane gas for a given time. Hence, the biogas stream can represent between 0 and 100% of the total methane gas stream sent to be LMG production plant and the proportion of the alternative source of methane gas consequently varies between 0 and 100% as well. The gas supply must accept all possible scenarios and must also mix gases from two or more sources to create the required mixed methane gas feed stream.
Mixing together two or more different streams of gas is not always a simple task and a designer can be faced with many challenges, especially when the proportion of each gas stream can vary greatly. Existing gas mixers normally mix a percentage of a secondary gas into a predominant primary gas.
One more challenge is the mixing two sources of methane gas that are under different conditions. For instance, the various gas streams may have different temperatures prior to their mixing. Mixing natural gas and biogas will generally occur at a lower pressure compared to the supply pressure of the natural gas. The pressure drop resulting from the gas expansion will cause the temperature of the natural gas to decrease significantly. However, since biogas is normally saturated with water vapor, mixing the cold natural gas stream with a biogas stream can cause condensation of water vapor present in the biogas stream. The condensate will need to be separated from the mixed methane gas feed gas stream. Still, snow or even ice may form under certain conditions, such as when the pressure drop of the natural gas is relatively important and when the mass flow rate of the biogas is not sufficient to keep the temperature of the mixed methane gas feed stream above the freezing point. Such situation can result in a blockage and even interrupt the methane gas stream to the LMG production plant. It is thus desirable to prevent such situation from happening.
Another desirable feature would be to have a gas supply that is generic enough to operate under a wide range of different conditions without the need of extensive modifications.
Accordingly, there is still room for many improvements in this area of technology.